Sometimes they hold their nose, sometimes yelling Geronimo (actually, Herbie only yelled it in Forbidden Worlds #114).
Adding to the power of the plummet, of course, is that Herbie is often flying and therefore falling a long way.
Many of the butt jokes in Herbie comics are preceded by a gravity assist.

Note Herbie's technique: Arms and legs outstretched (one arm, if holding nose), falling butt-first, maintaining that position through sound-effect-annotated impact, often causing squishing. Only recommended for the indestructible.